Conventionally, a small sized and thin type image pickup apparatus is mounted in a mobile terminal which is a small sized thin type electronic device, such as a mobile phone or PDA (Personal Digital Assistant). Hereby, not only an audio information but also an image information can mutually be transferred to a remote place.
As an image pickup element used for these image pickup apparatus, a solid state image pickup element such as CCD (Charge Coupled Device) type image sensor or CMOS (Complementary Metal-Oxide Semiconductor) type image sensor is used.
Recently, following the increase of spreading of these mobile terminals, a device in which the image pickup apparatus using a image pickup element with the large number of pixels is mounted is in the market in order to obtain an image with higher image quality. In this image pickup apparatus, corresponding to the image pickup element with the large number of pixels, there is an apparatus including the image pickup lens formed by a plurality of lenses in order to increase the resolution power.
As an image pickup lens used for a small sized high performance image pickup apparatus provided with such a image pickup element with the large number of pixels, an image pickup lens formed by 4 lenses is proposed because it can be made into higher performance as compared with the image pickup lens formed by 2 lenses to 3 lenses.
As this image pickup lens formed by 4 lenses, the so-called inverted Ernostar type image pickup lens, which aims the high performance and is provided with the first lens having a positive refractive power, the second lens having a negative refractive power, the third lens having a positive refractive power, the fourth lens having the positive refractive power, in the order from the object side, is disclosed (for example, refer to Patent Document 1).
Further, so-called telephoto type image pickup lens which is provided with the first lens having the positive refractive power, the second lens having the negative refractive power, the third lens having the positive refractive power, and the fourth lens having the negative refractive power, in the order from the object side, and aims the down-sizing of the total length of the image pickup lens (the distance on the optical axis from the aperture stop to the image side focal point), is disclosed (for example, refer to Patent Document 2).
[Patent Document 1] Tokkai No. 2004-341013
[Patent Document 2] Tokkai No. 2002-365530
However, the image pickup lens written in the above Patent Document 1 has a disadvantage for a miniaturization, because it is the inverted Ernostar type which has the positive fourth lens, has a principal point placed in the image side of the optical system and has a longer back focus compared with a structure having the negative fourth lens as telephoto type structure. Furthermore, the lens having the negative refractive power is only one in the four lenses. So it is difficult to correct Petzval's sum, and it is difficult to secure the fine performance in the peripheral part of an image plane.
Further, the image pickup lens written in the above Patent Document 2 is the photographing field angle view is narrow, and further, the correction of the aberration is insufficient. So, there is a problem for the purpose to correspond to the high pixelation of the image pickup element.
Furthermore, when all of lenses forming the image pickup lens are formed by plastic lenses molded by the injection molding for having priority to the cost, it is advantageous for the down-sizing and cost-reduction of the image pickup lens. However, because the plastic material has a large refractive index change at the time of the temperature change, the image pickup lens has a disadvantage such that the image point position of the whole body is varied by the temperature when all of lenses are formed of plastic lenses. In the fixed focal point type apparatus, so-called pan-focus type image pickup apparatus, there is a problem that the variation of the image point position at the time of the temperature change can not be disregarded.